Thermal magnetic current limiting circuit breakers are well known in the prior art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 3,943,316, 3,943,472, 3,943,473, 3,944,953, 3,946,346, 4,612,430, 4,618,751, and 5,223,681 which are assigned to the same assignee as the present application, and which are hereby incorporated by reference. Basically, a current limiting circuit breaker comprises a base and cover, a stationary contact, a movable contact secured to a rotatable blade, arc interrupting chamber, an operating mechanism for opening and closing the contacts, and a trip unit which releases the operating mechanism when a predetermined amount of current is exceeded.
Trip systems are designed to respond to power faults detected in the circuit breaker. The above-mentioned circuit breakers employ a thermal magnetic trip systems to trip the circuit breaker in response to short circuit or overload faults. The thermal magnetic trip systems utilize a magnetic field to trip the circuit breaker. When the current level increases beyond a predetermined threshold, the magnetic field "trips" a mechanism which causes a set of circuit breaker contacts to release, thereby "breaking" the circuit path. The trip system also employs a bi-metal for detecting thermal overload faults. The extent of the bi-metal's deflection represents an accurate thermal history of the circuit breaker. Generally, the heat generated by the current overload will cause the bi-metal to deflect which initiates the tripping mechanism to break the circuit path.
Thermal magnetic trip systems are generally adequate for many simple circuit breaker applications, but there has been an increasing demand for a more intelligent and flexible tripping system. For example, many factories today include 3-phase power equipment which is often replaced or moved on a regular basis. Consequently, the circuit breaker tripping specification, e.g. current thresholds, for that equipment must be adjusted. Thus, processor-based tripping systems have been developed to provide user-programmable flexibility while maintaining the ability to interrupt the current path in response to fault conditions. These electronic systems employ micro-processor based electronic circuit boards such as the one described in U.S. Pat. No. 5,038,246, issuing on Aug. 6, 1991 to Durivage, III, entitled, "Fault Powered, Processor Controlled Circuit Breaker Trip System Having Reliable Tripping Operation" and is incorporated herein by reference.
Trip solenoids are typically employed in electronic trip circuit breakers for initiating the tripping of the circuit breaker. A representative trip solenoid is shown in U.S. Pat. No. 4,731,692, issued on Mar. 15, 1988 to Square D Company, entitled "Circuit Breaker Trip Solenoid Assembly" and includes an electronic circuit board which generates a trip signal that energizes the trip solenoid which, in further, then actuates the tripping mechanism of the circuit breaker. The disclosure in the '692 patent is incorporated herein by reference.
The trip solenoids in the prior art could cause part breakage when the trip solenoid plunger engaged with other circuit breaker components to initiate the tripping mechanism. In particular, trip solenoids employed plungers that did not allow any over-travel, thereby creating the potential for the plunger to damage other circuit breaker components. A need exists for a trip solenoid assembly that adequately performs the circuit breaker tripping action without creating the environment that could potentially damage circuit breaker components.